1. Field of the Invention
The present invention relates to a transmitting apparatus, receiving apparatus, communication system, and multiplex timing compensation method and, more particularly, to a transmitting apparatus, receiving apparatus, communication system, and signal multiplex timing compensation method which control a shift in input timing between a plurality of signals to a multiplexing unit, which occurs due to a wiring delay.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) method is known as a method of transmitting digital data.
According to the OFDM method, a plurality of carrier waves (to be referred to as “subcarriers” hereinafter) which are orthogonal to each other are prepared in a predetermined transmission bandwidth. By using inverse Fourier transform, the subcarriers are modulated (OFDM-modulated) at once on the basis of digital information to be transmitted. The modulated subcarriers are synthesized, i.e., multiplexed, and the multiplexed signal (OFDM signal) is transmitted.
In the OFDM method, Fourier transform is used to demodulate the OFDM signal obtained by OFDM modulation. In the OFDM method, pilot symbols (pilot samples) which are used even as information to, e.g., correct the Fourier transform start timing are sometimes inserted between data symbols (patent reference 1: JP 2003-510952).
FIG. 17 is a block diagram for explaining an OFDM communication system described in patent reference 1. The OFDM communication system described in patent reference 1 will briefly be described below with reference to FIG. 17. The OFDM communication system shown in FIG. 17 uses N subcarriers.
Referring to FIG. 17, a pilot symbol inserter 101 receives a data symbol stream and inserts pilot symbols to the symbols of the received data symbol stream at a predetermined interval.
A serial-parallel converter 103 separates the symbol output from the pilot symbol inserter 101 into N samples and parallelly supplies the separated samples to an inverse fast Fourier transformer (to be referred to as an “IFFT” hereinafter) 105.
The IFFT 105 receives the N samples output from the serial-parallel converter 103, executes inverse fast Fourier transform (to be referred to as “inverse Fourier transform” hereinafter), i.e., OFDM modulation to generate an OFDM symbol, and outputs the generated OFDM symbol to a guard interval (to be referred to as “GI” hereinafter) inserter 107.
The GI inserter 107 inserts a guard interval before the OFDM symbol to generate an OFDM signal.
A digital/analog converter (to be referred to as a “DAC” hereinafter) 109 converts the OFDM signal output from the GI inserter 107 into an analog OFDM transmission signal and transmits it.
The transmitted OFDM transmission signal is received by an analog/digital converter (to be referred to as an “ADC” hereinafter) 111.
The ADC 111 converts the received OFDM transmission signal into a digital OFDM signal containing a GI and N OFDM samples and outputs the digital OFDM signal to a GI remover 112.
The GI remover 112 removes the GI contained in the received OFDM signal and outputs an OFDM symbol containing the N OFDM samples.
The ADC 111 and GI remover 112 are operated by a predetermined timing error estimation signal.
A fast Fourier transformer (to be referred to as an “FFT” hereinafter) 114 receives the OFDM symbol containing the N OFDM samples, executes fast Fourier transform (to be referred to as “Fourier transform” hereinafter), i.e., OFDM demodulation for the N received OFDM samples, and outputs the N demodulated samples.
The N samples output from the FFT 114 are converted into a symbol by a parallel-serial converter 115 and provided to a pilot symbol detector 116.
The pilot symbol detector 116 detects a pilot symbol from the symbol output from the parallel-serial converter 115 and outputs the detected pilot symbol to a timing compensator 117. The pilot symbol detector 116 also outputs the symbol without the pilot symbol, i.e., a data symbol.
Upon receiving the pilot symbol from the pilot symbol detector 116, the timing compensator 117 obtains a timing error on the basis of the received pilot symbol, compensates for the obtained timing error, and outputs a timing error estimation signal to the ADC 111 and GI remover 112.
The timing compensator 117 will be described below.
The timing compensator 117 obtains the difference between a reference phase known in advance and the phase of the pilot symbol detected by the pilot symbol detector 116 and estimates the timing error by using the change ratio of the value. Note that a technique is known, which measures a delay profile from a pilot symbol and estimates a timing error on the basis of the measured delay profile.
In some transmitting apparatuses for transmitting information by the OFDM method, a plurality of OFDM signals are multiplexed and, more specifically, time-divisionally multiplexed in an analog manner, and the time-divisionally multiplexed signal is transmitted to a receiving apparatus by reason of apparatus implementation.
FIGS. 18A and 18B are block diagrams respectively showing a transmitting apparatus and a receiving apparatus which communicate by using a multiplexed OFDM signal.
Referring to FIGS. 18A and 18B, a transmitting apparatus 1201 includes a first signal generation unit 103, second signal generation unit 104, and multiplexing unit 105. FIGS. 18A and 18B show only two signal generation units. However, the number of signal generation units is not limited to two and may be three or more.
A first signal generation unit 103 generates a first OFDM signal SS0(1). The second signal generation unit 104 generates a second OFDM signal SS0(2). A plurality of subcarriers (carrier waves) in the first OFDM signal SS0(1) are identical to those in the second OFDM signal SS0(2).
The multiplexing unit 105 multiplexes the first OFDM signal SS0(1) and second OFDM signal SS0(2) in an analog manner. More specifically, the multiplexing unit 105 time-divisionally multiplexes the first OFDM signal SS0(1) and second OFDM signal SS0(2). The multiplexing unit 105 transmits the time-divisionally multiplexed signal as a transmission signal STX. The multiplexing unit 105 loads the plurality of received signals at a common timing and time-divisionally multiplexes the plurality of signals loaded at the common timing.
A receiving apparatus 1202 comprises a demodulation unit 107 and data reproduction unit 108.
The demodulation unit 107 receives a reception signal SRX corresponding to the transmission signal STX, executes OFDM demodulation for the received reception signal SRX, and outputs a demodulated signal SDMO.
The data reproduction unit 108 executes symbol determination for the demodulated signal SDMO and outputs the symbol determination result as a demodulated data sequence SRDAT.
When a plurality of OFDM signals are to be multiplexed and, more specifically, time-divisionally multiplexed in an analog manner, the input timing of the plurality of OFDM signals to the multiplexing unit varies in some cases. This variation occurs on the basis of, e.g., the difference in cable length between the OFDM signal generation units and the multiplexing unit or the individual difference between the signal generation units.
If the input timing of the plurality of OFDM signals to the multiplexing unit varies, the multiplexing timing by the multiplexing unit also varies.
FIGS. 19 and 20 are explanatory views for explaining the shift of multiplexing timing by the multiplexing unit. The same reference numerals as in FIGS. 18A and 18B denote the same parts in FIGS. 19 and 20.
FIG. 19 shows the state of a time-divisionally multiplexed signal 105a output from the multiplexing unit 105 when the second OFDM signal SS0(2) output from the second signal generation unit 104 is supplied to the multiplexing unit 105 with a delay Δt with respect to the first OFDM signal SS0(1) output from the first signal generation unit 103.
FIG. 20 shows the state of a time-divisionally multiplexed signal 105b output from the multiplexing unit 105 when the second OFDM signal SS0(2) output from the second signal generation unit 104 is supplied to the multiplexing unit 105 simultaneously with the first OFDM signal SS0(1) output from the first signal generation unit 103.
The second OFDM signal SS0(2) contained in the time-divisionally multiplexed signal 105a shown in FIG. 19 has the delay Δt with respect to the first OFDM signal SS0(1) as compared to that contained in the time-divisionally multiplexed signal 105b shown in FIG. 20.
When data is to be acquires by demodulating the second OFDM signal SS0(2) contained in the time-divisionally multiplexed signal 105b shown in FIG. 20, a data acquisition error caused by Δt can occur. If a data acquisition error occurs, the communication quality becomes poor.
The problem of poor communication quality based on the variation of multiplexing timing by the multiplexing unit rises not only when OFDM signals are time-divisionally multiplexed but also when the multiplexing unit time-divisionally multiplexes a plurality of signals.